A fresh look at glacial floods

We tend to think of continental-scale ice sheets as rather ponderous affairs, inexorably advancing southward over the landscape and then slowly retreating to the north at the end of each ice age. Over the last 20 years, however, evidence has accumulated that this is a misconception. We now know that the Laurentide Ice Sheet—the largest ice-age glacier—was characterized by thin, marginal ice streams flowing rapidly on low-friction beds and was unstable through much of its history (1–3). The ice sheet periodically and abruptly discharged massive amounts of ice into the North Atlantic (4), and abrupt coolings and warmings occurred throughout the last ice age (5).